1. Field of the Invention
The present invention relates to an optical information processing apparatus for irradiating a focused light beam on a recording medium to record or reproduce information. In the present invention, the term optical information processing apparatus includes a record-only apparatus, a reproduction-only apparatus and a recording and reproducing apparatus.
2. Description of the Prior Art
In the past, an optical disk memory and a magneto-optical disk memory have been known optical information processing apparatus. In such optical information processing apparatus, a light beam modulated by an information signal irradiates a record medium. In the optical disk, a reflection factor at an area irradiated by the light beam changes, and in the magneto-optical disk, a magnetization direction at an area irradiated by the light beam changes. In reproducing the information, a light beam of a predetermined intensity is irradiated to the information bearing record medium and a reflected light or transmitted light modulated by the information is photo-electrically converted by a photo-sensor.
Such a record medium is characterized by a high information recording density. A track width of the information and a spacing between the tracks are very narrow. As a result, in the optical information processing apparatus, a focusing control for aligning a focus point of the light beam to a surface of the record medium and a tracking control for precisely directing the light beam onto the track on the record medium are essential. A focus point detection method used in the prior art apparatus of this type is explained below.
A method used in an apparatus of FIG. 1 is usually called an astigmatism method. A light beam (which vibrates horizontally to a plane of the drawing and is a p-polarized to a polarized beam splitter 3) emitted from a semiconductor laser 1 is collimated by a collimeter lens 2, passes through the polarized beam splitter 3, circularly polarized by a .lambda./4 plate 4 and focused onto a record medium 6 by an object lens 5. The light beam is reflected by the record medium 6, passes through the object lens 5, s-polarized by the .lambda./4 plate 4 and reflected by the polarized beam splitter 3. The light is focused by a condenser lens 7 and a cylindrical lens 8. The cylindrical lens 8 has a unidirectional focusing function. As the position of the record medium 6 vertically shifts, a shape of the focused light beam deforms orthogonally with respect to an in-focus position b, as shown by a and c. The deformation of the shape is detected by a four-division photo-detector (not shown) to produce a focusing error signal by which the focusing control is effected.
In this method, since the photo-detector must be precisely positioned in two axial directions in a plane normal to an optical axis, the positioning of the photo-detector is difficult to attain. If the object lens is moved perpendicularly to the optical axis for tracking control, the focused light beam is also moved perpendicularly to the optical axis and a focusing error is produced. The complex cylindrical lens is required for the astigmatism. This leads to an increase in cost.
FIG. 2 shows a focus point detection method called a knife edge method. A construction on the left of a focusing lens is similar to that shown in FIG. 1 and hence it is omitted in FIG. 2. A light beam separated from an incident light by the polarized beam splitter is focused to a position 11 by the focusing lens 9. A screen plate for rendering the light beam asymmetric is arranged between the focusing lens 9 and the position 11. If an in-focus state of the incident light changes on the record medium, a light beam directed to the focusing lens changes from the collimated light beam to a focused light beam or a scattering light beam. Accordingly, the shape of the focused light beam at the position 11 changes among a', b' and c' where b' represents an in-focus state. The deformation of the shape is detected by a two-division photo-detector (not shown) to produce a focus point error signal by which the focusing control is effected.
In this method, since the focus point is detected by the change of shape of the light beam, the photo-detector must be precisely aligned uniaxially in a plane normal to the optical axis. Thus, the positioning of the photo-detector is hard to attain.